In fluid mechanics and aerodynamics research, much valuable information can be gained from visualizations of both the outer flow and surface shear stress patterns over solid bodies immersed in fluid streams. The liquid crystal coating method, i.e., a method wherein a liquid crystal coating is applied to a surface under test, is a diagnostic technique that can provide areal visualizations of instantaneous shear stress distributions on surfaces in dynamic flowfields with a response that is rapid, continuous and reversible. Reference is made, for example, to Klein, E. J., "Liquid Crystals in Aerodynamic Testing," Astronautics and Aeronautics, Vol. 6, July 1968, pp. 70-73.
Liquid crystals are highly anisotropic "fluids" that exist between the solid and isotropic liquid phases of some organic compounds. As such, these crystals exhibit optical properties characteristic of a crystalline (solid) state, while displaying mechanical properties characteristic of a liquid state. Typically, in flow-visualization applications, a mixture of one part liquid crystals to nine parts solvent (presently, Freon) is sprayed on the aerodynamic surface under study. A smooth, flat-black surface is essential for color contrast and must be kept free of grease and other chemical contaminants. Recommended applications (after spray losses) are about 10 to 20 ml liquid crystals, measured prior to mixing with the solvent, to each square meter of surface area. The solvent evaporates, leaving a uniform thin film of liquid crystals whose thickness, based on mass conservation and estimated spray losses, is approximately 10 to 20 .mu.m (0.0004 to 0.0008 inch). Once aligned by shear, the molecules within the liquid crystal coating selectively scatter incident white light as a spectrum of discrete colors, with each color at a discrete angle (orientation) relative to the surface. For "thermochromic" liquid crystal compounds, this molecular structure, and thus the light scattering capability of the coating, responds to both temperature and shear stress. For newly-formulated, shear-stress-sensitive/temperature-insensitive compounds, such as Hallcrest BCN/192, "color play" (i e., discerned color changes at a fixed angle of observation, for a fixed angle of illumination) results solely from the application of shear stress.
While it is now known that this technique can be calibrated (under carefully controlled conditions) to measure surface shear stress magnitudes, two important issues remain: time response and "directional sensitivities," i e. sensitivity to illumination and viewing angles, as well as to the instantaneous shear stress direction.
The time-response issue has been investigated (see Parmar, D. S., "A Novel Technique for Response Function determination of Shear Sensitive Cholesteric Liquid Crystals for Boundary Layer Investigations," Review of Scientific Instruments, Vol. 62, No. 6, June 1991, pp. 1596-1608.) by placing a liquid crystal layer (about 100 .mu.m thick) between two optical glass plates and applying known and transient shear forces via a displacement of one plate. Liquid crystal delay, rise and relaxation time constants were measured as a function of the monochromatic wavelength of the incident light. Time constants in the range of 10 to 100 milliseconds were generally observed, with minimum values being on the order of 3 milliseconds. The extrapolation (or applicability) of these results to actual fluid mechanic applications, wherein liquid crystal coating thicknesses are an order of magnitude less, remains an open question.
Another approach to characterizing the time-response and/or flow-direction-indication capabilities of the liquid crystal technique is to expose the coating to transient viscous flows of known time scales and/or known shear directions. Oscillating airfoil experiments provided some initial results concerning both issues (see Reda, D. C., "Liquid Crystals for Unsteady Surface Shear Stress Visualization," AIAA Paper 88-3841, July 1988 and Reda, D. C., "Observations of Dynamic Stall Phenomena Using Liquid Crystal Coatings," AIAA Journal, Vol 29, No. 2, February 1991, pp. 308-310.)
In general, while previous investigators have utilized the liquid crystal technique, the focus of such research has been on attempting to determine parameters related to the magnitude of shear stress, for example the sudden increase in shear stress levels at the transition from laminar to turbulent flow. Reference is made, for example, to U.S. Pat. No. 4,774,835 (Holmes) and to Holmes et al, "A New Method for Laminar Boundary Layer Transition Visualization in Flight-Color Changes in Liquid Crystal Coatings," NASA Technical Memorandum, 87666, January 1986.